1. Field of the Invention
This invention relates to drive belt tensioning apparatus and, more particularly, to a device having a pivotable tensioner arm which is biased against a drive belt to produce a predetermined tension thereon and to a damping mechanism for controlling pivoting of the tensioner arm to avoid tensioner arm backlash and the generation of detrimental system vibrations.
2. Background Art
It is conventional to simultaneously operate automobile accessories, such as pumps, alternators, air conditioning systems, etc., by a single endless drive belt that is configured in a serpentine arrangement around the ends of drive shafts for those accessories. For the accessories to operate consistently and effectively, it is necessary for the belt to be maintained at a predetermined tension. This controlled tensioning is conventionally accomplished by biasing an idler pulley against an unsupported portion of the belt between drive and driven pulleys. The bias may be developed by hydraulic pressure, air pressure, a rubber spring, a steel spring, a resin spring, or the like. Belt tension is controlled by moving the belt in tension, compression, or in a twisting manner.
An exemplary prior art belt tensioner is shown in U.S. Pat. No. 4,285,676, to Kraft. In Kraft, a tensioner arm 27 is attached for pivoting movement relative to a fixed support. The tensioner arm 27 carries at its end a rotatable idler pulley 32 which, as seen in FIG. 1, is borne against an unsupported portion of a drive belt 2 between a drive pulley 4 and a driven pulley 5. The tensioner arm 27 has a mounting hub 28 which surrounds a shaft 21 and rotates relative thereto. Torsional coil springs 38, 39 surround the hub 28 and act between the torsion arm and fixed support to exert a rotational bias on the tensioner arm 27 to thereby bear the idler pulley 32 with a predetermined force against the belt 2.
It is the objective of designers of belt tensioning systems, such as that in Kraft, to afford a system that responds abruptly to belt slackening to prevent interruption of accessory operation by reason of belt slippage and/or disengagement of the belt from the accessories. However, while it is desirable to increase belt tension abruptly, it is also desirable to gradually reduce belt tension applied through the tensioning apparatus in the event that there is an abrupt increase in belt tension, as at start-up or upon rapid acceleration of a vehicle engine. Belt designers thus contend with the competing objectives of rapidly increasing belt tension in response to belt slackening and limiting kickback of an idler pulley and associated tensioner arm in response to an abrupt tensioning of the belt.
The Kraft structure achieves the first objective, but not the latter. If the belt portion between the drive and driven pulleys in Kraft is abruptly tensioned so as to overcome the predetermined biasing force produced by the tensioner arm, the belt will cause kickback of the tensioner arm 27.
One attempted solution to this problem is disclosed in U.S. Pat. No. 4,473,362, to Thomey et al. Thomey et al use a brake band 61 to create friction that varies depending upon the loading of the coil spring 20. The Thomey et al system is relatively complicated to construct. Further, the damping force will change in the event that there is a variation in the characteristics of the spring 20. Further, coil springs tend to deform as they are loaded by twisting, which results in uneven and generally unpredictable damping on the tensioner arm.
Other structures which damp tensioner arm rotation with a force controlled by a coil spring, which also pivotably biases the tensioner arm, are shown in U.S. Pat. Nos. 4,473,362, to Thomey et al, and 4,557,707, to Thomey.
Another vexatious problem in the drive belt industry is the development of detrimental vibrations that occur in drive systems, such as that described above. This problem is particularly prevalent in systems wherein a plurality of shafts are driven by a long drive belt. A resonant condition may result in the belt and/or the tensioning apparatus. Due to abrupt changes in angular velocity in automobile engines, severe vibrations of the belt may occur, resulting in generation of noise, variation in the tension on the belt developed through the belt tensioner, or disengagement of the belt altogether from the surface(s) which it drives.
The resonant condition in the belt tensioner may result in fluctuation of the belt tension as well as variation in the torque produced by the belt on the load/accessories. A further adverse effect of this vibration is that the belt may intermittently disengage from the accessories which it is driving. This may occasion uneven belt wear and premature failure of the belt.
In spite of the problems of belt and tensioner system vibration, few prior art structures have taken any measures to prevent such vibrations.